Refactor: libstonithd: Query stonith devices more clearly

Previously, we performed an XPath search that gets all "agent"
attributes. But for XPath search results that are not element nodes,
getXpathResult() returns an element (or NULL). In the case of an
attribute, it returns the attribute's parent element.

This is not at all clear. So instead, we now directly search for
elements that possess an "agent" attribute.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>